So confused

OK SO I WAS WITH MY PARTNER FO 5 YEARS, WE HAVE 3 KIDS AND I THOUGHT EVERYTHING WAS GOING OK, WE HAD THE ODD ROW LIKE MOST NORMAL COUPLES BUT THESE WERE ALWAYS RESOLVED WITHIN THE NEXT DAY, ANYWAY HE STARTED WORKING AWAY AND WOULD PHONE EVERYDAY, THAT WAS UNTIL I FOUND OUT HE WAS CHEATING ON ME THROUGH SOME REALLY RUDE MESSAGES ON HIS PHONE WHICH HE LEFT OUT.
STUPIDLY I SAID I WOULD FORGIVE HIM, BUT TO MY CONFUSION HES TURNED ROUND SAYIN IT WAS OVER A LONG TIME AGO AND JUST COULDNT BEAR TO TELL ME! NOW HIS WITH THIS OTHER LASS IN A DIFFERENT COUNTY AND IM LEFT LOOKING AFTER THE KIDS ONLY GETTING A BRAKE WHEN HE CALLS UP TO SEE THEM ONCE EVERY 2 WEEK!
AS YOU MIGHT GUESS I AM STILL TOTALLY IN LOVE WITH THIS GUY BUT JUST CANT UNDERSTAND HOW HE CAN MOVE ON SO QUICK, AND CARE SO LITTLE CAN ANYBODY HELP EXPLAIN?